Several devices which control a pump power source in response to some property of the pumping system are commercially available. Each of the different control devices monitors either directly or indirectly the quantity of liquid being pumped and disconnects the pump power source whenever the liquid being pumped falls below a predetermined value.
One group of control devices uses flow sensors which incorporate impellers, paddles, or differential pressure devices to measure the liquid flow rate from the pump at the wellhead. Mechanical devices like these tend to fail in service because of corrosion, scale build up and blockage by debris.
Beam monitor and rod tension devices are also used. Strain gauges are attached to the pump jack beam and mechanical or electronic tension gauges known as "dynomometers" are attached to the pumping rod or its support members to determine the stress on the rodstring as a function of time. The shape of the stress versus time graph is related to the quantity of liquid lifted by the down hole pump.
Some of the control devices use sonic sounding devices. An acoustical impulse is propagated down the annular region between the tubing and well casing. The return echo is analyzed to determine the height of the liquid in the annulus. The height and rate of change of height of the liquid column is used to determine a suitable pumping cycle.
Some control units use temperature sensors to determine the temperature of the polished rod. When the flow of liquid from the wellhead begins to decrease, the polished rod is no longer adequately cooled and its temperature increases because of frictional heating. The rising temperature of the polished rod is detected and used as a control parameter.
Some control units determine the rate at which electrical energy is consumed by the pump power source since the amount of power used depends on the quantity of liquid being pumped.